1. Field of the Invention
This invention relates to a process for regenerating a deteriorated iron-antimony oxide containing catalyst having the crystalline structure of an iron antimony compound and being substantially free of antimony trioxide.
2. Description of the Prior Art
It is known that an iron-antimony oxide containing catalyst composition comprising as four essential components (i) iron, (ii) antimony, (iii) at least one element selected from the group consisting of vanadium, molybdenum and tungsten and (iv) tellurium is useful as a catalyst for the oxidation, ammoxidation or oxidative dehydrogenation of olefins. Such catalysts have been found useful in the oxidation of propylene to produce acrolein, in the ammoxidation of propylene to produce acrylonitrile, in the oxidation of isobutene to produce methacrolein, in the ammoxidation of isobutene to produce methacrylonitrile and in the oxidative dehydrogenation of butene-1 or butene-2 to produce butadiene. For further details of the catalyst, reference is hereby made to the following U.S. patents to the present inventors et al., U.S. Pat. No. 3,668,147, U.S. Pat. No. 3,716,496, U.S. Pat. No. 3,988,359 and U.S. Pat. No. 4,083,804 incorporated herein by reference.
The present invention is directed primarily to regenerating iron-antimony oxide containing catalysts which have been deteriorated as a result of their use in the oxidation, ammoxidation or the oxidative dehydrogenation of olefins, however, it will be readily apparent to the skilled artisan that the present invention is equally applicable to catalysts deteriorated in other types of reaction.
These catalysts exhibit good catalytic performance both in terms of their activity and their continued activity (the useful life of the catalyst), but gradually a decrease in the activity of the catalyst is inevitable. Improper reaction conditions may accelerate the loss in catalytic activity.
It is economically undesirable to continue to use the catalyst once its activity has been reduced to below a critical level. The adverse effects of the reduced activity are multiplied in a larger scale production, for instance, a large scale production of acrylonitrile, and considerable economic loss will result unless the catalyst is replaced. However, the catalyst is expensive and it is expensive for the manufacturer to replace the deteriorated catalyst with a fresh one. It would therefore be a great economical advantage if a feasible method of regenerating the catalyst could be found.
As will be understood from the above explanation, one criterion for determining whether a catalyst is deteriorated or whether the deteriorated catalyst has been regenerated by a regenerative method on an economic, as opposed to a technical level, takes into account the activity and selectivity of the catalyst. Accordingly, based on experience, a catalyst is termed "deteriorated" if the yield is reduced by more than 2 to 3% of the yield when the catalyst was fresh, and a catalyst is termed "regenerated" if such yield is restored to the fresh yield or a higher level.
It is extremely difficult to enumerate the causes of the loss of activity experienced during use of the catalyst. In most cases, many factors combine to cause such deterioration. What is more, locating a particular contributing factor does not necessarily lead to the development of an effective method for regenerating the catalyst. Therefore, many attempts at providing an effective means of regenerating the catalyst have turned unsuccessful.
Various studies have been made on the method of regenerating a catalyst used for synthesis of unsaturated nitriles, unsaturated aldehydes and diolefins, but these studies do not appear to have been successful. A rare exception is a method of regenerating an antimony-uranium oxide catalyst described in Japanese Patent Application (OPI) No. 8615/72 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".) corresponding in part to British Pat. No. 1,365,096. That method is characterized by heating an antimony-uranium oxide catalyst complex in a fluidized state, in a non-reducing gas at a temperature of from 800.degree. to 1800.degree. F. and for a time such that the surface area of the catalyst does not fall below a minimum critical level of 5 square meters per gram. The basic concept behind the method is to heat the catalyst before its performance, which is determined by the surface area of the catalyst, drops to a minimum critical level. In addition, the method is applicable over a very wide range of temperatures. Therefore, it may well be considered a satisfactory and practical method. It even allows for regeneration in the reactor, as illustrated by some of the examples set forth in the specification. However, iron-antimony oxide containing catalysts cannot be regenerated by such simple method. As described hereinafter, only those iron-antimony oxide containing catalysts which have specific compositions can be regenerated under relatively limited conditions. Therefore, unless proper care is used during the regeneration of this type of catalyst, not only does the regeneration fail but the performance of the catalyst may be impaired and result in irreparable damage.
A novel process has previously been proposed in U.S. Pat. No. 4,049,575 for the production of catalysts that include the catalyst which is to be regenerated by the process of this invention. The process is an extremely effective production method and can also be used to regenerate the deteriorated catalyst, as demonstrated by some of the working examples. However, the regeneration procedure taught in this patent is rather complex and costly because it comprises the steps of (1) preparation of the solution of catalytic components for impregnation, (2) impregnation of the deteriorated catalyst and (3) drying and calcining the impregnated catalyst. If the method is applied to the regeneration of the catalyst, new or already contained catalytic components can be excessively added, thus producing a regenerated catalyst of a different composition than the original. The method also relies upon a new catalytic component, thus producing a regenerated catalyst of a different composition than the original. What is more, changes in the reaction rate and optimum reaction conditions easily occur. For this reason, the regenerative method has not been found entirely satisfactory. On the contrary, in the regeneration of the present invention, a remarkable change of the reaction rate and a great variation of the suitable reaction condition are not caused. Also, it is possible to mix the catalyst regenerated in accordance with the present invention with a fresh catalyst. Therefore, the regeneration method of the present invention is commercially desirable.